The invention relates to rotary drill bits for drilling or coring holes in subsurface formations, and of the kind comprising a bit body having a shank for connection to a drill string, a plurality of circumferentially spaced blades on the bit body each extending outwardly away from the central axis of rotation of the bit, a plurality of cutters mounted side-by-side along each blade, and a passage in the bit body for supplying drilling fluid to the surface of the bit for cleaning and cooling the cutters.
In rotary bits of this kind, it is usual for the cutters on the various blades to be located at different radial distances from the bit axis so that the cutters together define a cutting profile which, in use, covers substantially the whole of the bottom of the bore hole being drilled. For example, it is common for the cutters to be so positioned on the blades that they form a generally spiral array so that the path swept by each cutter partly overlaps the paths swept by the cutters which are at slightly smaller and slightly greater radial distances from the bit axis.
Drill bits are also known in which the cutters are arranged in a number of generally concentric arrays so as to cut generally concentric annular grooves in the bottom of the bore hole, this being found to enhance the stability of the bit.
Generally speaking, in the case where the cutters are in a spiral array, the stability of the bit in the bore hole increases with increasing number of blades. Thus, a six-bladed bit will generally be more stable than a three-bladed bit, other things being equal. However, it is also found that a bit having a smaller number of blades may perform more efficiently and achieve higher penetration rates, particularly in softer formations. Accordingly, in some formations a three-bladed bit may drill at a fast rate, but it may have a tendency to become unstable, resulting for example in bit whirl, and the cutters on the bit may also tend to wear out rapidly since there is less redundancy of cutters to provide a back up and to share some of the shocks to which a drill bit is subjected in use. In order to overcome the latter problem, back up cutters or abrasion elements are sometimes mounted on each blade rearwardly of the cutters. However, in view of the close proximity of the back up elements to the cutters there may be poor cleaning and cooling of the back up elements and the increased width of the blade required to accommodate the back up elements may increase the frictional rubbing of the blades on the formation.
The present invention sets out to provide a novel form of drill bit which can provide the drilling efficiency of a bit having a smaller number of blades and cutters with the stability and wear resistance of a bit having a greater number of blades.